by Bill Keveney, USA TODAY

by Bill Keveney, USA TODAY

BURBANK, Calif. - For a show about genius physicists, The Big Bang Theory shows little understanding of television's laws of gravity.

The hit CBS comedy (tonight, 8 ET/PT) has strapped rocket boosters onto its ratings in its sixth season, up substantially in viewers and especially among the young adults advertisers prize. It's hitting highs at a time of life when the typical show is arcing toward earth and cancellation.

Big Bang is also the No. 1 show in syndication, and the two occurrences are not unrelated. Heavy local repeat broadcasts are combined with plentiful rerun offerings on cable's TBS (on Tuesdays, for example, between cable and many local stations, viewers can watch seven repeats over a four-hour period). Those repeats, which both began in 2011, are pulling in new viewers, who discover they like Bang and head to CBS for new episodes.

"You kind of have to handcuff yourself to a pole in the street to not ever see our show at this point, because it's on every second," says Simon Helberg, who plays engineer and newlywed Howard Wolowitz.

But the boom in Big Bang, created by sitcom vets Chuck Lorre and Bill Prady, likely is more than a simple syndication phenomenon. Over the seasons, the ensemble has expanded, adding new characters and story lines as well as depth and complexity, always a balancing act on an already-popular series. And this season the critically acclaimed comedy has been even more willing to show its heart.

"I think this is our strongest season. It's my personal favorite," says Johnny Galecki, who plays physicist Leonard Hofstadter, the more socially skilled half of the offbeat roommate duo at the nucleus of Bang. "To do the story arcs we're now doing, we didn't really do that too much in the first few seasons. And the relationship stories, it takes a while to earn that with an audience with comedy."

But the show doesn't push it too far, says Jim Parsons, who has won two Emmys for his portrayal of Leonard's roommate, the abrasive, exacting Sheldon Cooper. "I feel like (the writers) are very careful to keep it from getting sentimental. That would suck the laugh out of the room."

The characters certainly have evolved since the series started in 2007. Back then, it was Leonard and Sheldon; their pretty, more socially adept neighbor, Penny (Kaley Cuoco); and their brilliant, nerdy scientist friends, Wolowitz and Raj Koothrappali (Kunal Nayyar). The guys had Star Trek and comic books, but not much going on in the way of relationships.

Their universe has expanded. Leonard's on his second, and more grounded, go-round with Penny; self-styled ladies' man Wolowitz is now an astronaut married to microbiologist Bernadette (Melissa Rauch); and innocent, chaste Sheldon has a steady gal, neuroscientist Amy Farrah Fowler (Mayim Bialik). There's even hope for tongue-tied Raj, who recently met a woman, Lucy (Kate Micucci), whom he may be able to communicate with without having to be drunk.

But don't expect smooth sailing. "These guys are not just going to wake up one morning and be the coolest kids in town. They're going to progress, but they're going to progress slowly," Nayyar says. "These guys are getting older, too. And as you get older, your priorities are going to slightly shift. You are going to start thinking about family and love."

Parsons reminds, however, that it's taken six seasons, longer than the lives of most TV shows, to get this far. "It seems illogical, but what I think has made (the show) develop and given it depth is the painfully slow pace at which they are developing. I think that lends a realistic credence to them. These aren't major changes."

Time and the arrival of newer characters, such as Amy and Bernadette, have helped flesh out the originals, says executive producer Steven Molaro, who took over day-to-day operation of the show this season. ""If you told me in Season 1" that Wolowitz would be a married astronaut, "my reaction would have been, 'Well, that sounds ridiculous.' But when you watch it happen over the course of six years, it seems completely plausible and a beautiful thing to watch."

"You can take any two characters in this group and put them in a room, and something funny is going to happen, which is great," Cuoco says.

For Lorre, who has had more than his share of successes (e.g., Two and a Half Men, Grace Under Fire, Cybill) and headline-generating headaches (e.g., Two and a Half Men, Grace Under Fire, Cybill), Big Bang is unalloyed bliss.

"I don't take it for granted that a TV show should work so beautifully and joyfully on so many levels," he says. A show is "a very stressful environment. It's a family environment for many, many years and, like any family, things go wrong and things go right and I've seen both. And I certainly cherish when it goes right."

More pop and bang at beginning

There wasn't much big about Big Bang at its start. During its first season, it aired only eight modestly rated episodes before going on the shelf for four months due to the long writers' strike. That loss of time could have depleted a fledgling series, but the writers, cast and crew redoubled their efforts when they returned, Galecki says. "If anyone had a toe outside of the water before the writers' strike, we just all dove in as soon as we came back."

It worked. After averaging just 8.4 million viewers in that abbreviated first season, the ratings bang has gotten bigger each season save for the fourth, when it moved from Monday to Thursday, where it established a new night of comedy for CBS. This season, Big Bang is up 16%, averaging 18.8 million viewers, making it the No. 2 entertainment show in viewers and No. 1 in young adults (up 11%).

It regularly beats once-dominant American Idol, and a January episode crossed the 20 million-viewer threshold. It's also tops in syndication, averaging 11.2 million viewers a week.

The Comic-Con favorite has leveraged its popularity in the geekosphere, too. A new species of Brazilian orchid bee, Euglossa bazinga, was named for Sheldon's catchphrase; the character also has an asteroid (246247 Sheldoncooper) named after him. Physicist Stephen Hawking and astronaut Mike Massimino have guest-starred as themselves, and Leonard Nimoy has done a voiceover as Star Trek's Spock.

"It's wonderful," Lorre says. "Imagine that you're working on a sitcom and you look around and go, 'Oh, there's a Nobel Prize winner here on the stage today. And there's folks from NASA here today.' And, oh, by the way, yeah, Stephen Hawking will do your show. It's crazy, it's just thrilling. It's been an amazing experience to see the kind of people who want to come play with us."

There's a mutual respect. "Bill and I never set out to write a show about nerds. When we first developed The Big Bang Theory, what was interesting was, could we build a show around extraordinarily intelligent characters who nevertheless have difficulty adapting to daily life?" Lorre says.

A microcosm in one scene

Big Bang is taped before a studio audience on a sound stage at Warner Bros., but some scenes are shot in advance. During a "pre-shoot" for the episode airing next week, the gang comes over to Howard and Bernadette's place to talk about a letter left to Howard by the father who had abandoned him. Howard doesn't want to read it, but they know what it says. Each member of the group seeks to provide solace by offering Howard a different version of what the letter says.

The scene has science, something called "quantum superposition." It has humor: Sheldon's description is ridiculous and instantly perceived as false. And it has emotion: They all care for Howard and he is moved. "I think the writers do such a fantastic job balancing the humor and the heart," Rauch says. "The fact that Sheldon interjects something in the midst of all this heart-wrenching stuff is perfect."

Some scenes can be touching and funny in the same instant, as when Sheldon chooses Amy to be his emergency contact. "It still gets a ha-ha-ha when people saw that Sheldon made Amy the contact. (But) I think that's about the most transformative thing our characters could do for each other," Bialik says.

Prady credits Molaro for adding extra emotion. "These are stories that I think would have been uncomfortable in Season 2, but in Season 6, now that we know these characters so well, they feel right," Prady says.

Where these relationships will go from here is anybody's guess. The writers say they don't plan too far ahead, letting stories gain their own momentum. The actors say they just don't know what's going to happen.

Cuoco hopes that Penny and Leonard will be together at the end of the series, but that's just it - hope. "We could be broken up next week. It's always been so up in the air, which I've actually really liked about it because I feel like it's very true to life."

The actors won't predict how long the show will run. Save for Parsons, who offers a Spock-like specific: 10 years. "I may be wrong. I'm not saying it like a pat on the back. I'm just trying to be logical about it."

Galecki surmises the show is past the halfway point, but feels it hasn't hit that mark in terms of the stories left to tell. The actors say they want to stay on the ride until it ends.

"I'm still trying to grasp that we're here," Helberg says. "It's fun being on a show that in its sixth season is making headlines."